This invention generally relates to an apparatus for controlling the transport of a flexible web between a pair of spatially positioned spools and more particularly relates to such a mechanism which incorporates a self-reversing mechanism actuated when the web tension exceeds a predetermined amount.
Communication printers generally include ink ribbon devices for continually advancing the ribbon during printing. Continual movement of the ribbon distributes the ink usage over the entire ribbon thus greatly increasing ribbon life while improving the quality and clarity of the copy. It is conventional to draw the ribbon from a supply spool to a takeup until the ribbon in the supply spool is exhausted. The direction of ribbon feed is thereafter reversed and the original take-up spool becomes the supply spool.
Various types of ribbon drive mechanisms which include tension control reversing features are well-known. One particular device is described in U.S. Pat. No. 3,670,981 entitled "Self-Reversing Ribbon Drive" issued June 20, 1972 to J. J. Cavella. Cavella utilizes a pair of web-bearing spools which are alternately driven in opposite directions by a driver gear. The mechanism is shiftable between respective positions whereat the driver gear is drivingly coupled to one of the spools and then, in response to a predetermined ribbon tension, the alternate spool serves as the takeup spool thus reversing the direction of movement of the ribbon. To effectively eliminate the condition wherein the driving mechanism is lodged in an indeterminate position, mechanical biasing means in the form of magnets are utilized to assure that at least one of the spools is functioning as a motor driven take-up spool. The Cavella arrangement necessarily results in a rather complicated and relatively large mechanism and does not lend itself readily to use in compact printer assemblies which may be desired for mobile installations. Various other forms of ribbon reversal mechanisms have been described to accomplish the reversing by means of switches, relays or solenoids including a variety of various mechanical linkages. Many such arrangements have been proven to be costly, complicated, and relatively unreliable.
With respect to the illustrated embodiment, the ribbon is transferred from a supply spool to a take-up spool. In response to a tensioning condition of the ribbon occasioned by exhaustion of the ribbon from the supply spool or the presence of an undesirable jam condition, a shifting mechanism is actuated which reverses the drive to the spools and thus the direction of ribbon movement. Serving to prevent the mechanism from accidentally lodging in an intermediate position wherein neither of the two spools act as the take-up spool, the embodiment includes a detent mechanism which assures positive transfer of the driving force from one spool to the other upon sensing of a ribbon tension condition. As illustrated, the embodiment provides a unique and improved means for a ribbon reversing mechanism which is compact allowing its utilization in relatively small printer assemblies. Although the following description concerns transport of an ink ribbon, the principles of the invention are applicable to apparatus for handling other forms of flexible web.